boost levels during compression braking

Gothmolly

Veteran Member
Joined
Feb 3, 2005
Location
Providence, RI
TDI
2002 Golf
Can anyone with a modded ALH, with a boost gauge, tell me what their no-throttle boost levels are on the highway? Say you're driving along at 70 mph, and take your foot off the pedal - what's your boost levels when the car is driving the motor? I'm at about 4psi, wondering if that's common.
 

PakProtector

Veteran Member
Joined
Jan 5, 2014
Location
AnnArbor, MI
TDI
Mk.4's and the Cummins
Best I could do is examine a log or 6. I don't recall seeing anything so high though...going from memory.

Douglas
 

burpod

teh stallionz!!1
Joined
Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
various tunes do this differently... you would want to log 011-001 on a drive and do some revving up to 4000 in 3rd, or 3000+ in 4th, and then let it coast down in gear to see the target/spec boost + n75 on this. some tunes will call for the extreme - ton of boost and full vane closure. stock calls for very little boost. having a higher target boost off-fuel with corresponding correct n75 can reduce "shift-lag", but too much and if you're power shifting real quick you can get huge spikes going into the next gear. vanes closed will have a small amount of engine braking affect, and also have a tendency to keep intake air/exhaust temps higher. vanes open can have a little more shift-lag, but also lower IAT/EGT quicker off-fuel...

you might just have sticky vanes. all a guessing game without logging. if this behavior is something that has changed over time, it's probably sticky vanes or vac/n75 issues.

unfortunately the way off-fuel boost is controlled in the ecu is a little simplistic. it would be neat if there were some maps to control n75 with the positioin of the clutch switch + speed + fuel as well, but i don't think there are....
 

Nevada_TDI

Top Post Dawg
Joined
Aug 17, 2008
Location
Reno, sort of...
TDI
2001 Jetta TDI
C'mon @burpod, look deeper, work harder, get better results. j/k
I am happy with the tunes you have written me, and tomorrow or the next day I will get the boost leak resolved.
 

PradoTDI

Veteran Member
Joined
Sep 25, 2020
Location
MT
TDI
1991 Toyota LandCruiser LJ78 with ALH Swap
I’ve seen as high as 7-8 psi coasting down steep hill in 3rd gear.
 

burpod

teh stallionz!!1
Joined
Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
I’ve seen as high as 7-8 psi coasting down steep hill in 3rd gear.
some tunes i've seen call for basically max boost for coasting in gear (vanes 100% closed). i think that's excessive, but it might be what one wants for a paritcular application. iespecially if you quick shift with this kind of tune, you can get very huge boost boost spikes....
 

PradoTDI

Veteran Member
Joined
Sep 25, 2020
Location
MT
TDI
1991 Toyota LandCruiser LJ78 with ALH Swap
some tunes i've seen call for basically max boost for coasting in gear (vanes 100% closed). i think that's excessive, but it might be what one wants for a paritcular application. iespecially if you quick shift with this kind of tune, you can get very huge boost boost spikes....
Pretty sure that is the case with mine; I get a lot of exhaust braking effect and notice a distinct lack of low end torque until boost starts to build. Boost comes quickly once the engine starts to rev a bit, but then drops off drastically to 10-12psi once it hits around 18psi under moderate throttle or 22psi under heavy throttle. I have had to mess with my turbo actuator rod a number of times, most recently to replace it due to wear, and I'm sure it is not adjusted properly right now. Hopefully when I have my Mityvac back at the end of the week I can get it dialed in a little better.

I have been meaning to take some boost logs and post them to your thread, but can never seem to remember to take my computer along when I am out driving around. The exhaust braking is actually pretty nice when driving around town or descending steep hills off road, but having the vanes completely closed at low RPM makes steep hill starts a pain, especially when trying to climb a rock step or get some wheel speed up in deep snow or mud.
 

burpod

teh stallionz!!1
Joined
Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
Pretty sure that is the case with mine; I get a lot of exhaust braking effect and notice a distinct lack of low end torque until boost starts to build. Boost comes quickly once the engine starts to rev a bit, but then drops off drastically to 10-12psi once it hits around 18psi under moderate throttle or 22psi under heavy throttle. I have had to mess with my turbo actuator rod a number of times, most recently to replace it due to wear, and I'm sure it is not adjusted properly right now. Hopefully when I have my Mityvac back at the end of the week I can get it dialed in a little better.

I have been meaning to take some boost logs and post them to your thread, but can never seem to remember to take my computer along when I am out driving around. The exhaust braking is actually pretty nice when driving around town or descending steep hills off road, but having the vanes completely closed at low RPM makes steep hill starts a pain, especially when trying to climb a rock step or get some wheel speed up in deep snow or mud.
lack of low end torque is often a sign of too much advance (in the tune). this also causes (imo) huge boost spikes, followed by dips. it's not all the fault of the timing, but also just bad n75 map.. mismatched fueling maps etc... actuator length for just about all garrett vnt15-17ish (+ hybrids) should really be right around 18hg. it can be less, or it can be more (shorter/longer), as long as you are close to that and it's able to move the full range of motion, the rest can be adjusted in the tune (n75 map)

and yes, too much vane closure down low can hamper things, but it's generally all about a balance of having well adjusted fuel/timing/boost maps...

when things are right (tune + hardware - most of the fault is almost always in the tune), you should be able to plant your foot anywhere, anytime and get at *most* 2psi overboost. so you'd see just a bit of a spike but immediately settle and the rest of the pull should have calm steady boost...
 

PradoTDI

Veteran Member
Joined
Sep 25, 2020
Location
MT
TDI
1991 Toyota LandCruiser LJ78 with ALH Swap
I took a log of Group 11 and noticed that at idle my N75 was at 14.7%, so not fully closed I guess. I suppose that having an actuator rod out of adjustment could easily put it at an actual 0% though.

According to my boost gauge I don't see any dangerous boost spikes, almost never hit the 22psi that is the max for my tune. That said I never use WOT during normal driving, I think the only times I have buried the throttle was doing a data log a few years back, and then later on two dyno pulls.

Would advanced timing in the tune show up in the timing graph in VCDS? Last I checked my timing was slightly advanced, but still well within spec on the graph.
 

burpod

teh stallionz!!1
Joined
Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
14.7% should be 100% closed, unless rod length is 20+hg. you can check this and see, use a feeler gauge on the set screw while at idle ;)

the physical timing per the graph is only setting the physical limits of what's available to the tune. it really doesn't have much bearing on how advanced you're running, but it would depend on what tune.

not sure what tune you have, but for anything other than a rocketchip tune, you're going to want to make sure 04 timing adaptation is 32768.

to know what timing your'e doing, in a stock tune, you'd want to log 011-001-004, or if you don't care about seeing the boost at the same time, 011-004

if you log 004 you will see if actual is meeting spec, and what the spec is... it's about as important as if actual boost is meeting spec, except for the issue that you can't always be sure if the spec timing/boost is actually what's best ;)
 

PradoTDI

Veteran Member
Joined
Sep 25, 2020
Location
MT
TDI
1991 Toyota LandCruiser LJ78 with ALH Swap
My tune is a Malone Stage 4, tuned for 22psi max on a GT1749va turbo from a BHW. By next weekend I will have my Mityvac back and will be able to check my actuator rod length. The stop screw is an unknown as it came loose after installing the turbo and wound itself down until the actuator movement was severely limited. I brought it back to where I thought it was about right, but it could easily be off. Sometime this week I will do another log of 011 and 004 to check on timing.

Overall it drives pretty well, but I think there is potential for increasing mpg and cooling down my EGT's. After replacing the actuator rod I have noticed less seat of the pants kick when accelerating, so further adjustment there is definitely in order.

One thing I have been curious about would be to instal a VSV in the vacuum line going from the N75 to the actuator so that with a tune with less vane closure at low RPM I could still use exhaust braking when coasting down mountain passes or doing steep off road descents. Would that amount of boost differential be enough to trigger a CEL or limp mode? Another benefit would be to be able to fully open the vanes at idle for a quicker turbo cool down prior to shutting down the engine.

I will freely admit that I know very little about the electronics/tuning side of TDI's, most of my TDI's have been stock and most of the other diesels I have owned were fully mechanical.
 

burpod

teh stallionz!!1
Joined
Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
My tune is a Malone Stage 4, tuned for 22psi max on a GT1749va turbo from a BHW. By next weekend I will have my Mityvac back and will be able to check my actuator rod length. The stop screw is an unknown as it came loose after installing the turbo and wound itself down until the actuator movement was severely limited. I brought it back to where I thought it was about right, but it could easily be off. Sometime this week I will do another log of 011 and 004 to check on timing.

Overall it drives pretty well, but I think there is potential for increasing mpg and cooling down my EGT's. After replacing the actuator rod I have noticed less seat of the pants kick when accelerating, so further adjustment there is definitely in order.

One thing I have been curious about would be to instal a VSV in the vacuum line going from the N75 to the actuator so that with a tune with less vane closure at low RPM I could still use exhaust braking when coasting down mountain passes or doing steep off road descents. Would that amount of boost differential be enough to trigger a CEL or limp mode? Another benefit would be to be able to fully open the vanes at idle for a quicker turbo cool down prior to shutting down the engine.

I will freely admit that I know very little about the electronics/tuning side of TDI's, most of my TDI's have been stock and most of the other diesels I have owned were fully mechanical.
don't try and hack something in like that. it's never gonna work. retune is better ;)

this set screw is from a garrett vnt17. i think it might be a touch high. ~3 full threads should be about the max height afiact

 
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